The invention relates to a method and a device for controlling a drive train of a motor vehicle with a drive assembly and a continuously variable automatic transmission.
In modern motor vehicles familiar electronic control units, such as electronic transmission controls which can also be connected with electronic engine controls, are used for controlling the drive train.
Such electronic control units are also required for a continuously variable automatic transmission, which is also called a CVT (continuously variable transmission) automatic transmission, which contains a variable speed gear with a first taper disk set on a drive shaft as the primary disk set and with a second taper disk set on a driven shaft as the secondary disk set. Each taper disk set respectively consists of an axially fixed first disk and an axially displacable second disk which are described as a primary disk or as a secondary disk, depending on whether they are assigned to the primary disk set or secondary disk set. In order to be able to adjust the primary disk or the secondary disk, they are fed a pressurizing medium wherein the pressure that is respectively applied to the setting spaces of the primary disk and the secondary disk is determined, via an electronic control unit and electromagnetic positioning elements and hydraulic valves that are selected by this control unit.
In case of a malfunction of the electronic control unit, emergency programs are generally provided which are designed so as to be able to operate a vehicle in an emergency operating mode.
From EP 0 784 767 B1, we know of an emergency device for controlling a CVT wherein an electronic control device determines the pressure level of the particularly dynamically unbalanced setting spaces of the primary disk and the secondary disk via the electromagnetic positioning elements and the hydraulic valves. The emergency device, which is provided in the event of failure of the electronic control device, contains two pressure control valves, two pressure reduction valves and at least one emergency valve, wherein a primary valve and a secondary valve adjust a constant static pressure ratio and/or force ratio between the primary disk and the secondary disk with a constant static secondary pressure level.
The amount of these static force ratios and of the dynamic forces that are applied to the dynamically pressure non-compensated disks allows a determination as to how great the gear ratio change of the CVT is when transitioning from normal operation to the emergency operating mode. The emergency operating mode is described as a state of the CVT in which the governing speed and applied pressure control of the secondary disk are interrupted.
The problem with these hydraulic emergency programs lies in the control of the unavoidable hydraulic valve tolerances because a shift in the constant pressure ratio directly affects the gear ratio setting range.
From experiments we know that pressure tolerances of 5 bar can be present on the variable speed gear disks. The appropriate gear ratios in the emergency program are dispersed accordingly. For example, in the case of a tolerance setting with minimal pressure on the primary disk and maximum pressure on the secondary disk within the tolerance range, the gear ratio can be too far towards the smallest possible gear ratio (LOW) with a value considerably above 1, which drastically limits the permissible maximum speed during the emergency operating mode and wherein an abrupt change from a normal mode, at high speed, into the emergency operating mode can lead to a so-called over speeding of the driving motor, transmission damage and possibly a destruction of the transmission.
When considering a tolerance setting where a maximum pressure exists on the primary disk and a minimum pressure on the secondary disk within the tolerance range, a mean gear ratio level arises with a gear ratio in the direction of the largest possible gear ratio (overdrive, OD). A transition from the normal driving mode with a high vehicle speed into an emergency operating mode at this gear ratio level is not critical for the intactness of the transmission, however, the start-up power, despite excessive converted torque, may possibly be insufficient for starting to move the vehicle on slight inclinations after a standstill or a shut-down motor.
In order to prevent the transmission from being damaged when activating the emergency program due to too large a gear ratio or too small a gear ratio being set for starting to move the vehicle, valves can be used in which pressure is supplied in proportion to RPM or speed, e.g. through pitot tubes, however, this is associated with a considerably increased design complexity. The possible use of an engine speed program, which supplies information about the vehicle speed to the hydraulic emergency program, also proves problematic because such an engine speed program is no longer available when the electronic control unit, which adjusts the pressure ratio on the variable speed gear, fails and thus must be compensated for via a separate electronic control unit at additional costs.
The task of the present invention is to make a method and a device for controlling a drive train of a motor vehicle with an available continuously variable automatic transmission with which, in the event of a failure of at least one control unit that selects the continuously variable automatic transmission and a transition from normal operation to an emergency operating mode, a gear ratio is set in the variable speed gear of the continuously variable automatic transmission which avoids damage to the transmission and, at the same time, makes sufficient start-up power available.
To resolve this, the invention provides for the fact that in the event of failure of the electronic control unit, the frictional connection between the drive assembly and the continuously variable automatic transmission is interrupted before an emergency operating mode is made available through an emergency device by re-establishing the frictional connection.
As the device for controlling such a drive train with a continuously variable automatic transmission, which contains a variable speed gear, and an emergency valve device, which is activated upon failure of the electronic control unit, is provided in the pressure supply line to a clutch arrangement of a forward/reverse driving unit of the continuously variable automatic transmission, wherein the valve is switched into a position that separates the pressurized medium flow to the clutch arrangement, in the event of failure of the electronic control unit, so as to interrupt the frictional connection between the drive assembly and the continuously variable automatic transmission through an emergency program pressure.
The invented method and/or the invented device, respectively, allow the frictional connection in the drive train to be interrupted upon activation of the hydraulic emergency program, wherein beneficially independent from the tolerance setting of the disk valves of the variable speed gear, even an over speed of the driving motor is prevented when the electronic control unit fails at extremely high vehicle speed and the emergency device is activated.
On the other hand, the invented method and the invented device allow the frictional connection to be established for a gear ratio which is within the range of the smallest possible gear ratio (LOW) when re-starting the motor during the emergency operating mode, i.e., in a state of the continuously variable automatic transmission in which no governing speed and no applied pressure control occurs through the variable speed gear, so that sufficient start-up power is available and a maximum vehicle speed of well over 100 km/h is ensured, thus making a relatively large operating range available.